1. FIELD OF THE INVENTION
This invention relates to an improvement in an integrated spray drying and calcining process, particularly for kaolin, wherein hot exit gases from the calciner are used as a heat source for the spray dryer. In one specific aspect, it relates to a method for inhibiting corrosion of the equipment utilized in such a process.
Kaolin clay was one of the first inorganic materials to be dried commercially on a large scale using spray dryers. To prepare pure kaolin for the paper, paint, plastics, rubber and ceramic industries, crude kaolin from the mine is milled, crushed and slurried with water and chemical dispersants, and a fine clay slip containing about 25% solids is then separated. The slip is flocculated with sulfuric acid and further chemically treated to improve quality, centrifuged and filtered to obtain a slurry having a solids content suitable for spray drying. During spray drying of the slurry, heated air at a temperature of about 600.degree. C. is fed the spray drier, and gases at about 100.degree.-120.degree. C. exit the spray drier. The dried kaolin leaving the spray drier is at about 90.degree.-110.degree. C. and contains less than 1% free moisture and about 14% crystal bound water.
The kaolin from the spray drier is then passed to a calciner and calcined. The calcined kaolin leaving the calciner is at a temperature in the range of 865.degree.-1100.degree. C. and the gases exiting the calciner are at about 600.degree. C. Those exit gases are generally treated to recover particulates and to remove environmentally hazardous materials, including acid-forming gases, before being released to the atmosphere. Separate sources of fuel are generally used to supply heat to the calciner and to the spray dryer in kaolin processing operations.
2. Discussion of the Prior Art
It is known to utilize calciner off-gases to supply heat to a spray dryer. For example, U.S. Pat. No. 3,776,688 describes the operation of a rotating kiln plant for producing cement according to the wet process. The capacity of the kiln is increased and heat economy is improved by drying a part of the cement slurry in the spray dryer and introducing the spray dried material into the kiln. The spray drying is performed using exhaust gases from the kiln in an integrated operation.
U.S. Pat. No. 4,717,559 describes a method for producing a calcined kaolin pigment by feeding an aqueous slurry of kaolin to a spray dryer which is supplied with heat by means of the exit gases from the calciner mixed with heated air provided by a combustor. The method recovers both heat energy and calciner dust from the calciner exhaust gases. The exhaust gases from the calciner are described in a preferred embodiment as being in the range of 800.degree. F. (427.degree. C.) to 1000.degree. F. (538.degree. C.), preferably at about 1000.degree. F. (538.degree. C.). A gas inlet temperature to the spray dryer of 1100.degree. F. (593.degree. C.) and a outlet temperature of 200.degree. F. (93.degree. C.) are exemplified.
When processing kaolin in an integrated spray drying and calcining process, the slurry being fed to the spray dryer usually contains a variety of sulfur-containing entities, including aluminum sulfate, sulfonated dispersants and various acid moieties. Sulfur-containing entities are also present in the spray dryer solid product being fed to the calciner. During calcination, certain of the sulfur-containing entities decompose to form various gaseous products, including sulfur trioxide. That sulfur trioxide, in the presence of water vapor under ordinary conditions of operation, forms liquid sulfuric acid. As is well known, sulfuricaacid is highly corrosive, particularly at elevated temperatures.